In general, as a vulcanization accelerator of rubber, a thiuram-based, sulfonamide-based, mercaptobenzothiazole-based or other accelerators are used. A sulfonamide-based vulcanization accelerator is a delayed action type accelerator, which is believed to produce mercaptobenzothiazole and amine by dissociation of the N—S bonds by heat during vulcanization. It is known that the produced mercaptobenzothiazole acts as a vulcanization accelerator, while the amine plays an important role in accelerating the vulcanization reaction by coordinating with zinc oxide to activate the vulcanization system and by reaction with the vulcanization intermediate (see Non-Patent Document 1).
As opposed to this, the disulfide-based vulcanization agent, dibenzothiazole disulfide, produces mercaptobenzothiazole by dissociation of the S—S bonds due to heat, but has no ability to stimulate the vulcanization by amines, and therefore, it is reported to vulcanization is slow and vulcanization accelerating capability is inferior to sulfenamides. It may be considered to jointly use amines for the purpose of improving the vulcanization accelerating ability of dibenzothiazole disulfide, but in such a case, since the reactivity of the free amines is high, there is the problem that they would react with the sulfur and other vulcanization agents even at a low temperature whereby a detrimental effect on the scorch time occurs.
Further, Patent Documents 1 to 6 describe various types of amine salts of carboxylic acid disulfides, but these documents described examples of using a quaternary ammonium salt of a carboxylic acid group-containing disulfide as a static electricity suppressant for a color toner for electrostatic charged image development (Patent Document 1), using a disulfide-containing amine salt as an aqueous ink composition for a ball point pen (Patent Document 2), using a mono- or di-amine salt of a dithiodipropionic acid or dithiodiglycolic acid as a rust preventing agent (Patent Document 3), a method for producing a quaternary ammonium salt of a carboxylic acid group-containing disulfide (Patent Document 4), using a quaternary ammonium salt of a carboxylic acid-containing disulfide as a toner for electronic photograph recording and a charge suppresser for a developer (Patent Document 5), and using a 3-mercaptopropionic acid disulfide as a water-soluble additive for a water-soluble functional fluid (Patent Document 6), but the art of introducing these salt compounds as a rubber vulcanization use compounding agent was not known. In particular, in the case of the quaternary ammonium salt of a carboxylic acid group-containing disulfide used in Patent Documents 1 and 4, since the amine ingredient acting to accelerate the vulcanization reaction is a quaternary amine, no substantial role of accelerating the vulcanization reaction can be expected.
Further, Patent Documents 7 and 8 disclose using a carboxyl group-containing disulfide as a vulcanization agent for rubber, but these are both carboxylic acid-containing disulfide compounds and not salt compounds containing an amine component having the role of accelerating the vulcanization reaction.
Further, a method for producing an amine salt of a polythiopolycarboxylic acid having 2 to 14 sulfur atoms is described in Patent Document 9. Patent Document 9describes, as prior art, European Patent Publication EP-A-0,780,429 (Patent Document 10) describing a method for producing di-, tri- and tetra-thiodipropionic acids. This production method has the defects that the content of the bonded sulfur is narrowly limited. Mixtures containing about 70% of dithiodipropionic acid and only about 30% of tri- and tetra-thiodipropionic acids are produced. As opposed to this, Patent Document 9 discloses a method for producing particularly pure polythiopolycarboxylic acid which enables the production of a compound containing a relatively high content of bonded sulfur. The examples of Patent Document 9 also disclose methods for producing polythiodipropionic acids having averages of four sulfur atoms and 3 to 11 sulfur atoms.
In general, sulfur compounds are classified by the number of sulfur atoms into mono- (one sulfur atom), di-(two sulfur atoms) and poly- (three or more sulfur atoms) sulfide compounds. This is because the dissociation energy of the sulfur bonds is about 70 kcal/mol in the case of disulfide and 45 kcal/mol in the case of trisulfide (three sulfur atoms) and, therefore, greatly differs according to the number of sulfur atoms and, when the number of sulfur atoms becomes 3 or more, the sulfur bonds easily dissociate (see Non-Patent Document 1). Therefore, a polysulfide compound is inferior to a disulfide compound in the heat stability, and, therefore, has the problem that it easily reacts with sulfur and other vulcanization agents or rubber etc. even at a low temperature and provides a detrimental effect on the processing process such as scorching of the rubber during mixing and shortening of the scorch time.
The bonding performance of the metal belt and rubber in a pneumatic tire is, of course, important from the viewpoint of the tire being a composite material. If this bonding performance is low, it leads to tire separation or other trouble. As a measure against this, the introduction of cobalt (Co) salts and changes in the vulcanization accelerator to make the bonding reaction better have been attempted (see Non-Patent Document 2), but this has the problem of a deterioration in the heat buildup property.
Non-Patent Document 1: Chapman, A. V., Porter, M.: “Sulphur Vulcanization Chemistry” in Natural Rubber Science and Technology, Roberts, A. D. Ed., Oxford Science Publications, London (1988).
Non-Patent Document 2: Journal of the Society of Rubber Industry of Japan, vol. 65, page 70 (1992)
Patent Document 1: Japanese Patent Publication (A) No. 2004-354708
Patent Document 2: Japanese Patent Publication (A) No. 2004-115684
Patent Document 3: Japanese Patent Publication (A) No. 11-92979
Patent Document 4: Japanese Patent Publication (A) No. 4-264063
Patent Document 5: Japanese Patent Publication (A) No. 6-501566
Patent Document 6: Japanese Patent Publication (A) No. 63-284294
Patent Document 7: Japanese Patent Publication (A) No. 2002-224244
Patent Document 8: Japanese Patent Publication (A) No. 9-262318
Patent Document 9: Japanese Patent Publication (A) No. 2001-89440
Patent Document 10: European Patent Publication EP-A-0.780, 429